An electrochemical measuring cell of the above kind is disclosed in U.S. Pat. No. 4,961,834 incorporated herein by reference. In this measuring cell, a measuring electrode, a reference electrode and a counter electrode are arranged in an electrolyte chamber of the measuring cell housing. The electrolyte chamber is filled with an electrolyte and the housing is closed off by a permeable membrane with respect to the measurement sample to be detected. The measuring electrode, the reference electrode and the counter electrode have respective connecting leads which pass through the measuring cell housing and are connected to an evaluation unit having a voltage source. A sensor current i(t) flows after the electrodes are connected to the voltage source.
It is a disadvantage of the known measuring cell that no information can be obtained from the sensor current i(t) as to the state of use of the measuring cell. Thus, the sensor current i(t) can lie within the predetermined limits but precise concentration measurements are no longer possible.
European patent publication 0,419,769 discloses a method for continuously monitoring an electrode system of potentiometric measurement cells wherein symmetrical bipolar current pulses having different period durations are applied repeatedly to the measuring cell. The voltage change caused thereby, referred to the electrode voltage without current pulse, is compared to a desired value determined experimentally or by computer.
It is a disadvantage of this known method that an additional voltage source is necessary with which the check is carried out. It is also disadvantageous that the check must be carried out at different times and with different period durations in order to detect the individual faults.